The manufacture of nonwoven webs has become a substantial part of the textile industry. There are a wide variety of uses for nonwoven webs, including the manufacture of surgical drapes, wiping cloths, carpets and components of disposable products such as diapers and sanitary napkins.
It is often desirable to incorporate an elastomeric web into a nonwoven fabric, particularly for nonwoven fabrics used in disposable garment and personal care products. Stretchable fabrics are desirable for use as components in these products because of their ability to conform to irregular shapes and to allow more freedom of body movements than do fabrics with limited extensibility.
There are a wide variety of techniques for producing nonwoven webs. Elastic nonwoven webs have been produced, for example, by meltblowing techniques. In meltblowing, thermoplastic resin is fed into an extruder where it is melted and heated to the appropriate temperature required for fiber formation. The extruder feeds the molten resin to a special meltblowing die. The die arrangement is generally a plurality of linearally arranged small diameter capillaries. The resin emerges from the die orifices as molten threads into a high velocity stream of gas, usually air. The air attenuates the polymer into a blast of fine fibers which are collected on a moving screen placed in front of the blast. As the fibers land on the screen, they entangle to form a cohesive web. Meltblowing forms very small diameter fibers, typically about two micrometers in diameter and several inches in length, which entangle in the web sufficiently so that it is generally impossible to remove one complete fiber from the mass of fibers or to trace one fiber from beginning to end.
Elastic meltblown webs exhibit a number of desirable properties. For example, the webs have good integrity due primarily to the fiber entanglement and surface attraction between the very small fibers. There are, in addition, advantages inherent in the meltblowing process itself. For example, the fibers are collected at a relatively short distance from the die, usually ranging from 12 to 6 inches, giving a positive control of the fiber blast and good edge control. Further, meltblowing can tolerate non-uniform polymer melts and mixtures of polymers which cannot be handled by other processes. A variety of polymers can be used in melt-blowing techniques, and in fact, melt blowing is said to be applicable to any fiber forming material that can give an acceptably low melt viscosity at suitable processing temperatures and which will solidify before landing on the collector screen.
Despite all of the advantages of meltblowing, however, there are several disadvantages to this technique for producing elastic nonwoven webs. The technique is inherently costly. The die configuration, essential to the production of meltblown fibers, requires a side-by-side arrangement of spinneret orifices. This limits the number of spinnerets that can be set up for production within a given area, which in turn limits both efficient use of floor space and the possible output of fibers. Further, preparing and monitoring the spinnerets is labor-intensive.
Meltblown webs are only moderately strong due to processing conditions. The meltblown polymer is molten during the entire fiber formation process, and due to the relatively short relaxation time of meltblown polymers, meltblown filaments typically are not highly oriented. Without the molecular alignment that occurs during more conventional fiber attenuation, and which lends strength to the fibers, the properties of elastic polymers are not optimized in meltblowing.
Meltblown webs also have less desirable aesthetic appeal. The noncontinuous network of fibers can give an unpleasant feel or "hand." Further, the network of fibers can snag and fiber shedding can be a problem.
There have been attempts to use the well known spinbonding process to produce elastic nonwoven fabrics. Various spinbonding techniques exist, but all include the basic steps of: extruding continuous filaments, quenching the filaments, drawing or attenuating the filaments by a high velocity fluid, and collecting the filaments on a surface to form a web. Spunbonded webs can have a more pleasant feel than meltblown webs because they more closely approximate textile filament deniers and consequently textile-like drape and hand.
One difference in the various spinbonding processes is the attenuation device. For example, in the Lurgi spinbonding process, multiple round or tube-shaped devices attenuate the filaments. A spinneret extrudes a molten polymer as continuous filaments. The filaments are attenuated as they exit the spinneret and are quenched, or solidified, by a flow of air. The filaments then enter the round attenuator gun where they are entrained with large quantities of high pressure air which provide the attenuation force for the filaments. As the filaments and air exit the gun, they move with an expanding supply of air to form a cone or a fan of separated filaments, which are deposited on a forming wire.
The use of round attenuator guns results in several problems. Tube-type attenuators consume large quantities of high pressure air, resulting in high utility costs and high noise levels. Additionally, these type attenuators must be individually strung up and monitored. If a filament breaks, the ends tend to plug the attenuator; the process must be stopped, the hole unplugged, and the filaments rethreaded. All of this results in decreased efficiency and increased labor.
Various slot draw processes have been developed to overcome the problems of the Lurgi process. In slot drawing the multiple tube attenuators are replaced with a single slot-shaped attenuator which covers the full width of the machine. A supply of air is admitted into the slot attenuator below the spinneret face with or without a separate quench step. The air proceeds down the attenuator channel, which narrows in width in the direction away from the spinneret, creating a venturi effect, and causing filament attenuation. The air and filaments exit the attenuator channel and are collected on the forming wire. The attenuation air, depending on the type of slot draw process used, can be directed into the attenuation slot by a pressurized air supply above the slot, or by a vacuum located below the forming wire.
Slot drawing has various advantages over the Lurgi process. The slot attenuator is self-threading in that the filaments fall out of the spin block directly into the slot attenuator. The high pressure air used by Lurgi devices is not always required, thereby reducing noise and utility costs. Further, the slot draw machines are practically plug-free. However, both the Lurgi and slot draw processes provide advantageous economics as compared to the melt blowing process.
In view of the advantages of the spinbonding processes, it would be desirable to provide elastic nonwovens by spinbonding. Attempts to impart elasticity to spunbonded fabrics, however, have been largely unsuccessful. One problem is breakage, or elastic failure, of the filaments during extrusion and drawing. Due to the stretch characteristics of elastomeric polymers, the filaments tend to snap and break while being attenuated in the molten or partially hardened state. If a filament breaks during production, the ends of the broken filament can either clog the flow of filaments or enmesh the other filaments, resulting in a mat of tangled filaments in the nonwoven web. Severe filament breaks manifest themselves as polymer droplets which are conveyed to the forming wire in the molten state causing tear outs and wire wraps.